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Avalanche Forecast

Archived

Dec 7th, 2017–Dec 8th, 2017

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely.
Below Treeline
Natural avalanches unlikely.

Regions

Olympics.

Watch for isolated wind slab pockets on lee terrain features on non-solar terrain.  Be especially wary of small loose wet avalanches near terrain traps. 

Detailed Forecast

Expect some high clouds and slightly cooler temperatures on Friday with light winds.

The avalanche danger should slightly decrease Thursday to Friday.

Recently formed, mostly small wind slab are most likely to linger on non-solar slopes. These wind slabs are expected to be stubborn or difficult to trigger, small and shallow. Watch for small areas of firmer, wind-transported snow on isolated, lee terrain features.

Snow on steep solar slopes should have most slid or consolidated in sunny warm weather on Thursday. Any high clouds and slightly cooler temperatures should diminish the danger of loose wet avalanches on Friday. But continue to watch for loose wet snow on steep solar slopes especially near sun warmed terrain features.

Early season terrain hazards still exist, such as poorly covered rocks, vegetation and creeks, particularly at lower elevations.

Snowpack Discussion

Warm, very wet weather before Thanksgiving caused wet snow and glide avalanches as well as snowpack consolidation, leaving a strong crust as a gift. Currently there were no snowpack concerns below the Thanksgiving crust and in general new snow received post-Thanksgiving has reportedly bonded well.

A series of frontal systems produced snow over the week following the Thanksgiving warm period: Hurricane Ridge received about 2 feet of snow.

Fair weather Sunday through Thursday will have allowed the layers to consolidate and heal significantly. On Thursday, temperatures warmed to 50's F at Hurricane.

Observations

NWAC Professional Observer Matt Schonwald was in the Hurricane Ridge area on Wednesday. South-facing terrain was warming significantly in due to solar warming and increasingly entraining deeper snow (up to 50-60 cm) as the day progressed. Some natural large (D2) loose wet slides were entraining enough snow to break branches on trees.

Matt also said that a previously unreported avalanche cycle on Sunday produced up to D2 skier-triggered and natural storm slab avalanches. On Wednesday, that same storm slab layer 20 cm down was still slightly reactive in compression on North-facing terrain, but does not propagate. Wind-deposits were shallow (less than 10 cm) on northerly terrain, with no ability to propagate and triggering possible in isolated terrain features.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.